This invention relates to down-the-hole drilling and provides improvements in the provision of flushing fluid in the vicinity of the bit head of a down-the-hole drill in order to facilitate the removal of rock chips from the hole. The fluid will normally be air but could be an air-liquid mixture or, in a purely hydraulic drill, liquid only.
In a down-the-hole drill, the hammer mechanism acts directly on a drill bit at the bottom of the hole and the forces on the bit are direct and large. It is therefore necessary to use a bit of considerable strength. Flushing fluid is supplied to the bit, often through a generally axial hole which extends through the shank of the bit to the face which acts on the floor of the hole, and sometimes through a series of grooves extending generally longitudinally along the shank of the bit and terminating at the face.
It is also known (from West German Pat. No. 1 238 864) to provide a pair of flushing holes in the bit, the holes leading from passages in the drill body to the top of the head and passing through the head. The holes are located opposite each other. It is however undesirable for the bit of a down-the-hole drill to be weakened by numerous internal cavities, so that a bit formed with two or more generally longitudinal holes through it is not as strong as may be wished. Two holes are however generally considered desirable where the bit is of the blade type, being provided because of the symmetry of the cruciform blade arrangement at the face of the bit. Apart from being structurally relatively weak, this arrangement also leads to undesirable turbulence and to closed air circuits at the bit head, which in turn lead to premature wear of the bit.
In another arrangement (known from U.S. Pat. No. 3,225,841) a central bore in the bit is provided, terminating in an orifice at the centre of the face. There is also a series of downwardly sloping passages extending from the central bore to the sides of the bit, where the passages end in grooves which direct the flushing air downwards to sweep the face of the bit. This arrangement suffers from the structural weakness inherent in bits with several internal cavities, and there is again considerable turbulence at the face, where the streams of flushing fluid tend to converge, and hence premature wear.
In out-of-the-hole drilling technology it is common to provide a longitudinal channel for flushing fluid along the drill stem or string which extends from the drill body to the bit. In such drills the head of the bit is usually considerably larger in diameter, compared to the diameter of the stem behind it, than is the case in down-the-hole drills, where the reduction in cross-sectional area of the apparatus directly behind the head of the bit is relatively slight since the casing housing the hammer mechanism is located immediately above the bit.
In out-of-the-hole drilling, the reduction in cross-section mentioned above has the consequence that the flushing fluid tends to expand into the space in the hole immediately behind the head, slowing down the overall flow. This has an unfavourable effect on the flushing action since the velocity of the flushing fluid should be maintained if the fluid is to perform a proper sweeping action.
To overcome this problem of out-of-the-hole drilling, it is known (for example from U.K. Pat. No. 1,071,418) to provide a central longitudinal bore through the drill bit, extending to the centre of the face, and also a series of further bores which extend from the central longitudinal cavity through the side wall of the bit, some being sloped towards the face and others away from the face. This arrangement divides the stream of flushing fluid and creates a venturi effect in which there is a high-pressure region at the face itself. Chips from the face are drawn from this area into a low-pressure region further up the hole where they are entrained in the rapidly moving stream of flushing fluid and conveyed up the hole.
In such drills the bit is naturally weakened by the presence of multiple bores and such bits would be inapplicable in down-the-hole drilling. Moreover the characteristics of the fluid available for flushing are entirely different compared with down-the-hole drills. The less marked difference in relative areas between the head and the zone behind the head also reduces the theoretical desirability of the venturi effect.